Structure changes of nanocrystalline mackinawite under hydrothermal conditions: formation of greigite and its structural properties

Structural changes and transformation of nanocrystalline mackinawite to greigite were investigated using powder X–ray diffraction measurements, Raman spectroscopy, X–ray absorption fine structure, and ab initio quantum chemical calculations. We clarify the formation conditions for greigite. Nanocrystalline mackinawite utilized by Sano et al. (2020) was used as a starting material. By hydrothermal heating at 140 °C for 12 h, nanocrystalline mackinawite was transformed into greigite without other iron phases. During the transformation, the crystallite size increased from 5 to 12 nm. After 3 h of hydrothermal heating, the Fe3+/ΣFe ratio markedly increased, but the a and c lattice parameters of nanocrystalline mackinawite were almost unchanged. Immediately after the transformation, greigite exhibited a larger lattice parameter, which suggests that excess Fe2+ was incorporated into the octahedral site. Thereafter, the greigite gradually shrunk and Fe3+/ΣFe progressively approached the ideal value. The changes in the unit cell volume and Fe valence state implied that when greigite initially occurred, the nonstoichiometry allowed to accommodate more Fe2+ in greigite to the octahedral site. The hydrothermal heating of nanocrystalline mackinawite is potentially applicable to the synthesis of pure nanocrystalline greigite.